NoClassDefFoundError: javax/servlet/http/HttpServletResponse異常解決方法

場景描述：
maven+springboot項目，啓動的時候出現異常，具體信息如下：

Caused by: java.lang.NoClassDefFoundError: javax/servlet/http/HttpServletRequest
	at java.lang.Class.getDeclaredMethods0(Native Method) ~[na:1.8.0_181]
	at java.lang.Class.privateGetDeclaredMethods(Class.java:2701) ~[na:1.8.0_181]
	at java.lang.Class.getDeclaredMethods(Class.java:1975) ~[na:1.8.0_181]
	at org.springframework.util.ReflectionUtils.getDeclaredMethods(ReflectionUtils.java:613) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:524) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	at org.springframework.util.ReflectionUtils.doWithMethods(ReflectionUtils.java:510) ~[spring-core-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.determineCandidateConstructors(AutowiredAnnotationBeanPostProcessor.java:247) ~[spring-beans-4.3.12.RELEASE.jar:4.3.12.RELEASE]
	... 17 common frames omitted
Caused by: java.lang.ClassNotFoundException: javax.servlet.http.HttpServletRequest
	at java.net.URLClassLoader.findClass(URLClassLoader.java:381) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:424) ~[na:1.8.0_181]
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:349) ~[na:1.8.0_181]
	at java.lang.ClassLoader.loadClass(ClassLoader.java:357) ~[na:1.8.0_181]
	... 24 common frames omitted

NoClassDefFoundError和ClassNotFoundException一般是運行時Class未加載到時出現的異常

ClassNotFoundException 1	NoClassDefFoundError 2
從java.lang.Exception繼承，是一個Exception類型	從java.lang.Error繼承，是一個Error類型
當動態加載Class的時候找不到類會拋出該異常	當編譯成功以後執行過程中Class找不到導致拋出該錯誤
一般在執行Class.forName()、ClassLoader.loadClass()或ClassLoader.findSystemClass()的時候拋出	由JVM的運行時系統拋出

那麼一定是運行時沒有加載到servlet-api.jar，由於是web項目用的tomcat部署，一般都是tomcat中；檢查maven配置

<groupId>com.geekq</groupId>
<artifactId>miaosha</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>jar</packaging>
    ...
  <dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-web</artifactId>
     <exclusions>
         <exclusion>
             <groupId>org.springframework.boot</groupId>
             <artifactId>spring-boot-starter-tomcat</artifactId>
         </exclusion>
         <exclusion>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-log4j12</artifactId>
         </exclusion>
         <exclusion>
             <groupId>log4j</groupId>
             <artifactId>log4j</artifactId>
         </exclusion>
     </exclusions>
 </dependency>
 ...
 <dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-tomcat</artifactId>
    <!--打包的時候可以不用包進去，別的設施會提供。事實上該依賴理論上可以參與編譯，測試，運行等週期。
        相當於compile，但是打包階段做了exclude操作-->
    <scope>provided</scope>
</dependency>

這裏把內置的tomcat生命週期配置成provided，造成運行時找不到class，調整後：

<dependency>
     <groupId>org.springframework.boot</groupId>
     <artifactId>spring-boot-starter-web</artifactId>
     <exclusions>
         <exclusion>
             <groupId>org.slf4j</groupId>
             <artifactId>slf4j-log4j12</artifactId>
         </exclusion>
         <exclusion>
             <groupId>log4j</groupId>
             <artifactId>log4j</artifactId>
         </exclusion>
     </exclusions>
 </dependency>